Introduction: Various seal species have been studied as experimental models for adaptation to asphyxia. Elephant seals (Mirounga angustirostris) were selected for study based on its ability to withstand extended periods of hypoxia. The spleen plays an significant role in the seal dive response and is believed to function as a reservoir for oxygenated erythrocytes. Elevated hematocrit levels during diving has led researchers to hypothesize that the spleen contracts gradually over the course of a dive, supplying the animal with oxygenated red blood cells. The goal of these studies was to determine the relationship between splenic contraction, hematocrit, and hepatic sinus volume during a diving response. Methods: Five 4 month old elephant seals were collected from Ano Nuevo, California and held at Long Marine Laboratory, UCSC. The animals were released at the conclusion of the experiment. The animals were transported to Stanford Univeristy on each day of an MR study. Each seal was placed in a restraining jacket, and a custom-made plexiglass helmet was placed on the seal and filled with water to ellicite a diving response. A single-shot fast spin echo pulse sequence yielded the best depiction of splenic anatomy. Baseline, diving, and post-dive scans were collected at one minute intervals. Each animal was dived 4 times (dive duration 5 to 8 min), resulting in a total of 20 complete diving data sets. Results: Analysis of the MRI data reveals a dramatic splenic contraction occurs immediately upon facial immersion of the animal. This contraction reduces the volume of the spleen to approximately one tenth of the recorded resting volume. Simultaneous enlargement of the haptic sinus is also observed. This response was observed during all 20 dives. Conclusions: Contrary to initial speculation on the pattern of splenic contraction, it appears that the spleen immediately transfers its contents to the hepatic sinus at the onset of a dive. We speculate that, through the action of the caval sphincter, these oxygenated erythrocytes are slowly metered into circulation via the vena cava, producing the observed gradual rise in hematocrit. Viewed in a larger context, these initial studies raises the possibility of important new insights into naturally evolved mechanisms of defense against recurring hypoxia, ischemia and reperfusion.